Diseases such as atherosclerosis and cancer are two of the leading causes of death and disability in the world. Atherosclerosis involves the development of fatty plaques on the luminal surface of arteries. These fatty plaques cause narrowing of the cross-sectional area of the artery. Ultimately, blood flow distal to the lesion is reduced causing ischemic damage to the tissues supplied by the artery.
Coronary arteries supply the heart with blood. Coronary artherosclerosis or coronary artery disease (CAD) is the most common, serious, chronic, life-threatening illness in the United States, affecting more than 11 million persons. The social and economic costs of coronary atherosclerosis vastly exceed those of most other diseases. Narrowing of the coronary artery lumen affects heart muscle resulting first in angina, followed by myocardial infarction and finally death, and more than three hundred thousand of those patients die before reaching the hospital. (Harrison's Principles of Internal Medicine, 14th Edition, 1998).
CAD can be treated using percutaneous transluminal coronary angioplasty (PTCA). More than 400,000 PTCA procedures are performed each year in the United States. In PTCA, a balloon catheter is inserted into a peripheral artery and threaded through the arterial system into the blocked coronary artery. The balloon is then inflated, the artery stretched, and the obstructing fatty plaque flattened, thereby increasing the cross-sectional flow of blood through the affected artery. The therapy, however, does not usually result in a permanent opening of the affected coronary artery. As many as 50% of the patients who are treated by PTCA require a repeat procedure within six months to correct a re-narrowing of the coronary artery. Medically, this re-narrowing of the artery after treatment by PTCA is called restenosis. Acutely, restenosis involves recoil and shrinkage of the vessel. Subsequently, recoil and shrinkage of the vessel are followed by proliferation of medial smooth muscle cells in response to injury of the artery from PTCA. In part, proliferation of smooth muscle cells is mediated by release of various inflammatory factors from the injured area including thromboxane A2, platelet derived growth factor (PDGF) and fibroblast growth factor (FGF). A number of different techniques have been used to overcome the problem of restenosis, including treatment of patients with various pharmacological agents or mechanically holding the artery open with a stent. (Harrison's Principles of Internal Medicine, 14th Edition, 1998).
Of the various procedures used to overcome restenosis, stents have proven to be the most effective. Stents are metal scaffolds that are positioned in the diseased vessel segment to create a normal vessel lumen. Placement of the stent in the affected arterial segment prevents recoil and subsequent closing of the artery. Stents can also prevent local dissection of the artery along the medial layer of the artery. By maintaining a larger lumen than that created using PTCA alone, stents reduce restenosis by as much as 30%. Despite their success, stents have not eliminated restenosis entirely. (Suryapranata et al. 1998. Randomized comparison of coronary stenting with balloon angioplasty in selected patients with acute myocardial infarction. Circulation 97:2502-2502).
Narrowing of the arteries can occur in vessels other than the coronary arteries, including the aortoiliac, infrainguinal, distal profunda femoris, distal popliteal, tibial, subclavian and mesenteric arteries. The prevalence of peripheral artery atherosclerosis disease (PAD) depends on the particular anatomic site affected as well as the criteria used for diagnosis of the occlusion. Traditionally, physicians have used the test of intermittent claudication to determine whether PAD is present. However, this measure may vastly underestimate the actual incidence of the disease in the population. Rates of PAD appear to vary with age, with an increasing incidence of PAD in older individuals. Data from the National Hospital Discharge Survey estimate that every year, 55,000 men and 44,000 women had a first-listed diagnosis of chronic PAD and 60,000 men and 50,000 women had a first-listed diagnosis of acute PAD. Ninety-one percent of the acute PAD cases involved the lower extremity. The prevalence of comorbid CAD in patients with PAD can exceed 50%. In addition, there is an increased prevalence of cerebrovascular disease among patients with PAD.
PAD can be treated using percutaneous transluminal balloon angioplasty (PTA). The use of stents in conjunction with PTA decreases the incidence of restenosis. However, the post-operative results obtained with medical devices such as stents do not match the results obtained using standard operative revascularization procedures, i.e., those using a venous or prosthetic bypass material. (Principles of Surgery, Schwartz et al. eds., Chapter 20, Arterial Disease, 7th Edition, McGraw-Hill Health Professions Division, New York 1999).
Preferably, PAD is treated using bypass procedures where the blocked section of the artery is bypassed using a graft. (Principles of Surgery, Schwartz et al. eds., Chapter 20, Arterial Disease, 7th Edition, McGraw-Hill Health Professions Division, New York 1999). The graft can consist of an autologous venous segment such as the saphenous vein or a synthetic graft such as one made of polyester, polytetrafluoroethylene (PTFE), or expanded polytetrafluoroethylene (ePTFE), or other polymeric materials. The post-operative patency rates depend on a number of different factors, including the luminal dimensions of the bypass graft, the type of synthetic material used for the graft and the site of outflow. Excessive intimal hyperplasia and thrombosis, however, remain significant problems even with the use of bypass grafts. For example, the patency of infrainguinal bypass procedures at 3 years using an ePTFE bypass graft is 54% for a femoral-popliteal bypass and only 12% for a femoral-tibial bypass.
Consequently, there is a significant need to improve the performance of stents, synthetic bypass grafts, and other chronic blood contacting surfaces and or devices, in order to further reduce the morbidity and mortality of CAD and PAD. For example, procedures that can cause radial enlargement of vessels (outward or positive remodeling) can compensate for progressive growth of atherosclerotic plaques, thus should postpone the development of flow-limiting stenosis.
With stents, the approach has been to coat the stents with various anti-thrombotic or anti-restenotic agents in order to reduce thrombosis and restenosis. For example, impregnating stents with radioactive material appears to inhibit restenosis by inhibiting migration and proliferation of myofibroblasts. (U.S. Pat. Nos. 5,059,166, 5,199,939 and 5,302,168). Irradiation of the treated vessel can cause severe edge restenosis problems for the patient. In addition, irradiation does not permit uniform treatment of the affected vessel.
Alternatively, stents have also been coated with chemical agents such as heparin, phosphorylcholine, rapamycin, and taxol, all of which appear to decrease thrombosis and/or restenosis. Although heparin and phosphorylcholine appear to markedly reduce thrombosis in animal models in the short term, treatment with these agents appears to have no long-term effect on preventing restenosis. Additionally, heparin can induce thrombocytopenia, leading to severe thromboembolic complications such as stroke. Therefore, it is not feasible to load stents with sufficient therapeutically effective quantities of either heparin or phosphorylcholine to make treatment of restenosis in this manner practical.
Synthetic grafts have been treated in a variety of ways to reduce postoperative restenosis and thrombosis. (Bos et al. 1998. Small-Diameter Vascular Graft Prostheses:Current Status Archives Physio. Biochem. 106:100-115). For example, composites of polyurethane such as meshed polycarbonate urethane have been reported to reduce restenosis as compared with ePTFE grafts. The surface of the graft has also been modified using radiofrequency glow discharge to fluorinate the polyterephthalate graft. Synthetic grafts have also been impregnated with biomolecules such as collagen. However, none of these approaches has significantly reduced the incidence of thrombosis or restenosis over an extended period of time.
The endothelial cell (EC) layer is a crucial component of the normal vascular wall, providing an interface between the bloodstream and the surrounding tissue of the blood vessel wall. Endothelial cells are also involved in physiological events including angiogenesis, inflammation and the prevention of thrombosis (Rodgers G M. FASEB J 1988;2:116-123.). In addition to the endothelial cells that compose the vasculature, recent studies have revealed that ECs and endothelial progenitor cells (EPCs) circulate postnatally in the peripheral blood (Asahara T, et al. Science 1997;275:964-7; Yin A H, et al. Blood 1997;90:5002-5012; Shi Q, et al. Blood 1998;92:362-367; Gehling U M, et al. Blood 2000;95:3106-3112; Lin Y, et al. J Clin Invest 2000;105:71-77). EPCs are believed to migrate to regions of the circulatory system with an injured endothelial lining, including sites of traumatic and ischemic injury (Takahashi T, et al. Nat Med 1999;5:434-438). In normal adults, the concentration of EPCs in peripheral blood is 3-10 cells/mm3 (Takahashi T, et al. Nat Med 1999;5:434-438; Kalka C, et al. Ann Thorac Surg. 2000;70:829-834). It is now evident that each phase of the vascular response to injury is influenced (if not controlled) by the endothelium. It is believed that the rapid re-establishment of a functional endothelial layer on damaged stented vascular segments may help to prevent these potentially serious complications by providing a barrier to circulating cytokines, preventing the adverse effects of a thrombus, and by their ability to produce substances that passivate the underlying smooth muscle cell layer. (Van Belle et al. 1997. Stent Endothelialization. Circulation 95:438-448; Bos et al. 1998. Small-Diameter Vascular Graft Prostheses:Current Status Archives Physio. Biochem. 106:100-115).
Endothelial cells have been encouraged to grow on the surface of stents by local delivery of vascular endothelial growth factor (VEGF), an endothelial cell mitogen, after implantation of the stent (Van Belle et al. 1997. Stent Endothelialization. Circulation 95:438-448.). While the application of a recombinant protein growth factor VEGF in saline solution at the site of injury induces desirable effects, the VEGF is delivered after stent implantation using a channel balloon catheter. This technique is not desirable since it has demonstrated that the efficiency of a single dose delivery is low and produces inconsistent results. Therefore, this procedure cannot be reproduced accurately every time.
Synthetic grafts have also been seeded with endothelial cells, but the clinical results with endothelial seeding have been generally poor, i.e., low post-operative patency rates (Lio et al. 1998. New concepts and Materials in Microvascular Grafting: Prosthetic Graft Endothelial Cell Seeding and Gene Therapy. Microsurgery 18:263-256) due most likely to the fact the cells did not adhere properly to the graft and/or lost their EC function due to ex-vivo manipulation.
Endothelial cell growth factors and environmental conditions in situ are therefore essential in modulating endothelial cell adherence, growth and differentiation at the site of blood vessel injury. Accordingly, with respect to restenosis and other blood vessel diseases, there is a need for the development of new methods and compositions for coating medical devices, including stents and synthetic grafts, which would promote and accelerate the formation of a functional endothelium on the surface of implanted devices so that a confluent EC monolayer is formed on the target blood vessel segment or grafted lumen thereby inhibiting neo-intimal hyperplasia.
In regard to diseases such as cancer, most therapeutic agents used to date have generalized systemic effects on the patient, not only affecting the cancer cells, but any dividing cell in the body due to the use of drugs in conventional oral or intravenous formulations. Yet in many cases, systemic administration is not effective due to the nature of the disease that is in need of treatment and the properties of the drug such as solubility, in vivo stability, bioavailability, etc. Upon systemic administration, the drug is conveyed by blood circulation and distributed into body areas including normal tissues. At diseased sites, the drug concentration is first low and ineffective which frequently increases to toxic levels, while in non-diseased areas, the presence of the drug causes undesired side effect. In certain instances, drugs are readily susceptible to metabolic degradation after being administered. Therefore, drug dose is often increased to achieve pharmacological efficacy and prolong duration, which causes increased systemic burden to normal tissues as well as cost concern for the patient. In other instances, the therapeutic potential of some potent drugs cannot be fulfilled due to their toxic side effects.
Therefore, much effort has been made to improve efficacy and targeting of drug delivery systems. For example, the use of liposomes to deliver drugs has been advantageous in that, in general, they increase the drug circulation time in blood, reduce side effects by limiting the concentration of free drug in the bloodstream, decrease drug degradation, prolong the therapeutic effect after each administration, reduce the need for frequent administration, and reduce the amount of drug needed. However, liposome systems that are currently available show limited efficiency of delivering drugs to target sites in vivo. See Kaye et al., 1979, Poznansky et al. 1984, U.S. Pat. Nos. 5,043,165, and 4,920,016.
To yield highly efficient delivery of therapeutic compounds, viral vectors able to incorporate transgenic DNA have been developed, yet the number of successful clinical applications has been limited. Despite the number of successes in vitro and in animal models, gene transfer technology is therefore proposed to marry with cell therapy. The ex vivo transfer of gene combinations into a variety of cell types will likely prove more therapeutically feasible than direct in vivo vector transfer. See Kohn et al., 1987, Bilbao et al., 1997, and Giannoukakis et al. 2003.
More recently local drug delivery vehicles such as drug eluting stents (DES) have been developed. See U.S. Pat. Nos. 6,273,913, 6,258,121, and 6,231,600. However, drug eluting stents of the prior art are limited by many factors such as, the type of drug, the amount of drug to be released and the amount of time it takes to release the drug. Other factors which need to be considered in regards to drug eluting stents are the drug interactions with other stent coating components, such as polymer matrices, and individual drug properties including hydrophobicity, molecular weight, intactness and activity after sterilization, as well as efficacy and toxicity. With respect to polymer matrices of drug eluting stents, one must consider the polymer type, polymer ratio, drug loading capability, and biocompatibility of the polymer and the drug-polymer compatibility such as drug pharmacokinetics.
Additionally, the drug dose in a drug eluting stent is pre-loaded and an adjustment of drug dose upon individual conditions and need cannot be achieved. In regard to drug release time, drug eluting stents instantly start to release the drug upon implantation and an ideal real-time release cannot be achieved.
It is therefore a long-felt need to develop an efficient systemic and local drug delivery system to overcome limitations of current available techniques. The present invention provides a system for the delivery of therapeutic agents locally or systemically in a safe and controlled manner.